1. Field of the Invention
This invention relates to a laser beam depicting apparatus, and is particularly suitable for application to a laser beam depicting apparatus for depicting patterns and to a laser beam working apparatus.
2. Related Background Art
In a laser beam depicting apparatus of this type, use has been made of raster scanning as shown in FIG. 1 of the accompanying, drawings, in which a desired wiring pattern is directly depicted on a photosensitive member 2 to form a photo-mask which is a printing negative when the wiring pattern is to be printed on a printed substrate.
That is, a laser beam LB0 emitted from a laser source 3 is ON-OFF-modulated by an ultrasonic light modulator 4 in accordance with image data, and the modulated laser beam LB1 passes through a compressor lens system 5 to a light deflector 6 such as an ultrasonic deflector. The laser beam LB2 emerging from the light deflector 6 is compensated by having an angle of deflection matching the "tilting" error of each reflecting surface of a rotational polygonal mirror 7, and the laser beam LB2 after being compensated passes through an expander 8 to the rotational polygonal mirror 7.
The laser beam LB2 having entered the rotational polygonal mirror 7 is deflected and scanned by the rotating reflecting surfaces of the rotational polygonal mirror, and the deflected and scanned laser beam LB3 passes through an f.theta. lens 9 and is raster-scanned at a uniform speed on the scanning surface of the photosensitive member 2 placed on a movable stage 10 in synchronism with the rotation of the rotational polygonal mirror 7.
Now, in effect, to ON-OFF-operate the light modulator 4 with high sensitivity, it is necessary to stop down the incident laser beam finely and for this purpose, a compressor lens system 11 is also provided on the incidence side of the light modulator 4 as shown in FIG. 2 of the accompanying drawings which shows the construction of an optical system.
The aforementioned tilting error of the rotational polygonal mirror 7 is the working error of the polygonal mirror and is representative of the degree of inclination of each surface of the polygonal mirror relative to the rotary shaft. When there is such a tilting error, the deflecting and scanning plane of the laser beam LB3 deviates and the pitch of the scanning lines does not become constant. The light deflector 6 controls the angle of deflection of the laser beam LB2 in synchronism with the scanning of each surface of the rotational polygonal mirror 7 to thereby correct it so that no irregularity of scanning may occur.
By moving the movable stage 10 in synchronism with the raster scanning in this manner, a two-dimensional image can be depicted on the surface of the photosensitive member 2.
When an image is to be depicted on a photosensitive member 2 of relatively high sensitivity such as a lith film, a laser producing a laser beam LB0 of single wavelength is used as the laser source 3.
However, when an image is to be depicted on a photosensitive member 2 of relatively low sensitivity such as dry film resist laminated on a copper layered plate such as a printed substrate, the laser light of single wavelength is insufficient in energy density and therefore, a method of producing a laser beam LB0 of plural wavelengths by the use of a laser of the type oscillating plural wavelengths at the same time, such as an argon ion laser, and thereby enhancing the energy density of a laser beam LB3 applied to the photosensitive member 2 has been proposed.
However, the light modulator 4 and light deflector 6 in the prior-art apparatus are essentially elements utilizing the diffraction phenomenon of light and therefore, firstly, where the laser beam on the incidence side includes light of plural wavelengths, there occurs so-called color dispersion in which the optical path differs depending on the wavelength of the emitted laser beam and thus, there occurs color misregistration to the light spot of the laser beam applied to the photosensitive member, and this has led to the problem that a clear and minute image cannot be obtained
Secondly, light energy is concentrated on the light modulator 4 by the compressor lens system 11, and this leads to the undesirable possibility that thermal destruction of the light modulator 4 may occur.
As a method of compensating for such color dispersion, I have proposed a method as shown in FIG. 3 of the accompanying drawings wherein a prism 12 is inserted forwardly of a compressor lens system 11 and a prism 13 is inserted forwardly of a compressor lens system 5, whereby a laser beam including plural wavelength components LB.sub.0X, LB.sub.0Y) and LB.sub.lX, LB.sub.lY) is color-dispersed in advance and extracted by means of the prisms 12 and 13 and is caused to enter a light modulator 4 and a light deflector 6, and laser beams LB1 and LB2 in which the plural wavelength components overlap each other are caused to emerge therefrom (Japanese Laid-Open Patent Application No. 61-193130).
However, in the construction as shown in FIG. 3, firstly, the light beams of all wavelengths are finely stopped down by the compressor lens system 11 and enter the light modulator 4 and thus, the light modulator 4 comprising an ultrasonic modulator may be thermally destroyed.
Secondly, the light deflector 6 is comprised of an ultrasonic deflecting element for varying the frequency of the input ultrasonic wave and changing the angle of diffraction of light to thereby deflect the laser beam and therefore, with the variation in the angle of deflection, the degree of color dispersion (i.e., the expanse of the beam of the component wavelengths separated by color dispersion) becomes different.
Accordingly, if within a range in which the tilting error of the rotational polygonal mirror 7 is small and the distribution band of the wavelength components of the laser beam LB0 is narrow, color dispersion can be compensated for by causing the color dispersion in advance by the prism 13, but if the tilting error of the rotational polygonal mirror 7 is great and the distribution band of the wavelength components of the laser beam LB0 becomes wide, for example, like an argon ion laser, the color dispersion can no longer be compensated for by the prism 13.
This results in creation of significant color misregistration on the photosensitive member 2 on which depiction is effected.